1. Field of the Invention
The present invention relates to a method for producing a titanium monophosphide layer on a titanium nitride layer. The present invention relates in particular to such a method that can be employed during the production of a multilayer metallization on a semiconductor material.
2. Description of Prior Art
It is known to deposit titanium monophosphide, TiP, at temperatures in the range of 1000xc2x0 C. in the presence of catalysts such as Au, Pt or Pd. The lowest temperature value described for the production of titanium phosphides is 750xc2x0 C.
Such a formation of TiP is described, for example, by S. Motojima, T. Wakamatsu, Y. Takahashi and K. Sugiyama, J. Electrochem. Soc., 123 (1976), 290. Said publication describes the whisker formation of titanium phosphides. Titanium monophosphide was formed from TiCl4 and PCl3 at temperatures from 850xc2x0 C. to 1150xc2x0 C. on catalysts such as Pt, Pd and Au. The deposition of thin layers of TiP is not disclosed in this publication.
It is generally known in the field of semiconductor technology to produce diffusion barriers between silicon and an aluminum metallization by reactive sputtering of titanium nitride (TiN). Typical layer thicknesses of the titanium nitride are about 50 nm. The disadvantage of sputtering is the inferior edge covering, in particular in narrow and deeply etched structures. CVD methods (CVD=chemical vapor deposition) provide improved edge covering.
As an alternative to deposition of the type indicated above, methods of forming titanium nitride are known which are based on the decomposition of organometallic precursors that already contain Tixe2x80x94N complexes. Furthermore, it is known to deposit titanium nitride using titanium tetrachloride (TiCl4) and ammonia (NH3). When decomposing organometallic precursors, the high carbon concentration built in as well causes problems. In the deposition of a titanium nitride layer using TiCl4 and NH3, the takeup of chlorine causes problems, which increases the resistivity of the barrier and may lead to corrosion of the aluminum. Furthermore, in case of multilayer metallization on a semiconductor, temperatures up to 450xc2x0 C. are admissible only. However, the TiCl4/NH3 reaction yields layers with a critical Cl concentration below 2 at % only at temperatures  greater than 550xc2x0 C.
In order to reduce in the titanium nitride layer the too high chlorine concentration produced in the TiCl4/NH3 process at 450xc2x0 C., it is known to reduce the built-in impurities of Cl atoms as well as incompletely reacted fragments of the precursors (xe2x80x9cTiNxClyxe2x80x9d) by subsequent (xe2x80x9cin-situxe2x80x9d) annealing. The resistivity of the titanium nitride layers correlates with the chlorine concentration, with the residual chlorine content being a measure for the completeness of the reaction. The reactive gas NH3 is known as annealing gas for the extraction of chlorine. By annealing the titanium nitride layer using NH3 at 450xc2x0 C. and for a duration of 120 seconds, the Cl content can be reduced to  less than 2 at %. The resistivity of this annealed layers has a value of  less than 300 xcexcxcexa9cm. It is thus considerably lower than in case of layers deposited by conventional CVD and a corresponding temperature.
As a mechanism for the extraction of chlorine in the abovementioned reaction gas annealing step, NH3 shifts the reaction equilibrium of the total reaction
6TiCl4+8 NH3xe2x86x926TiN+24HCl+N2
due to the high NH3 supply, in the direction towards the final products TiN+HCl.
The article xe2x80x9cPrecursor for the low-temperature deposition of titanium phosphide filmsxe2x80x9d by T. S. Lewkebandara et al. in: Chemistry of Materials, Vol. 7, No. 6, Jun. 1, 1995, Washington, US, pages 1053 and 1054, describes the deposition of a TiP film on a substrate of glass or silicon. To this end, the use of a low pressure chemical vapor deposition (LPCVD) is taught, making use of a complex of titanium tetrachloride and cyclohexylphosphine. Furthermore, the said article mentions the deposition of TiCl4 and PCl3.
Chemical Abstracts, Vol. 115, No. 16, Oct. 21, 1991, Columbus, Ohio, US, Abstract No. 164194 and New Technol. Electron. Packag., Proc. ASM Int. Electron. Mater. Process. Congr., No. 3 (1990), Ohio, US, pages 243-347, E. Kolawa et al.: xe2x80x9cAmorphous ternary thin-film alloys as diffusion barriers in silicon metallizationsxe2x80x9d, describes the production of a thin barrier layer of TiPN2 with the aid of a sputtering method.
It is an object of the present invention to provide a method for producing a titanium monophosphide layer on a titanium nitride layer.
In accordance with a first aspect of the present invention, this object is achieved by a method for producing a titanium monophosphide layer (TiP layer) comprising the following steps:
a) placing a substrate in a reactor;
b) depositing a TiN layer on the substrate by supplying TiCl4 and NH3 to the reactor; and
c) annealing the TiN layer while supplying PH3 to the reactor immediately after deposition of the TiN layer, so as to form the titanium monophosphide layer on the TiN layer.
A further object of the present invention consists in making available a diffusion barrier layer with improved properties between a semiconductor material, a dielectric or a ferroelectric material and a deposited metallization.
In accordance with a second aspect of the present invention, this object is achieved by a diffusion barrier layer between a semiconductor material, a dielectric or a ferromagnetic material and a metallization deposited thereon, consisting of a bilayer comprising a TiN layer and a TiP layer.
The present invention is based on the realization that a titanium monophosphide layer (TiP layer) can be formed on a titanium nitride layer (TiN layer) when a TiN layer deposited in a conventional process using titanium tetrachloride and ammonia is subjected, immediately after the deposition, to annealing in the same reactor, while phosphine (PH3) is supplied thereto. Phosphine is more active than NH3 as a donor for hydrogen and thus as a getter for Cl (PH3: xcex94Gf=13.6 kJ/mol; NH3: xcex94Gf=xe2x88x9216.4 kJ/mol). Phosphine at the same time can serve as a getter for oxygen.
Consequently, the present invention is based on the realization that cover layers of pure titanium monophosphide can be deposited preferably at 450xc2x0 C. by the annealing of layers of titanium nitride in phosphine. The phase TiP as well as the layer structure as a double layer or bilayer can be identified in unequivocal manner for example by means of X-ray diffractometers or by means of Auger electron spectroscopy. The TiN layers preferably are deposited on silicon wafers by an RTCVD process (RTCVD=Rapid Thermal Chemical Vapor Deposition) using titanium tetrachloride and ammonia. Annealing of the wafers in PH3 follows immediately thereafter in the same reactor.
The method according to the invention can be used advantageously in semiconductor technology in order to form a diffusion barrier layer between a semiconductor material, for example silicon, and a metallization, for example aluminum. The diffusion barrier then consists of a bilayer consisting of TiN and TiP.
The present invention thus provides, according to a further aspect, a diffusion barrier layer between a semiconductor material and a metallization deposited thereon, the diffusion barrier layer consisting of a bilayer comprising a TiN layer and a TiP layer. Such a diffusion barrier layer has numerous advantages as compared to a conventional diffusion barrier layer consisting of TiN.
The TiP coating layer produced has a very low resistivity as compared to TiN. Furthermore, it seals the entire TiN layer against the takeup of oxygen. The roughness of TiP is less than that of TiN. Furthermore, tensile stresses in the TiP coating layer are low in comparison with annealed TiN layers.
Further developments of the present invention are indicated in the dependent claims.